Crosslinked acrylic sheet with enhanced vacuum molding attributes

ABSTRACT

A protective film is placed on acrylic sheet. The film has a polyethylene side and an adherent EVA side; it will survive stressful lamination procedures as well as thermoforming, and can be easily removed from a finished article after thermoforming.

This is a division of application Ser. No. 964,999, filed Oct. 22, 1992,now U.S. Pat. No. 5,308,425.

TECHNICAL FIELD

This invention relates to acrylic sheets and laminates which haveimproved vacuum thermoforming properties. The improved properties areimparted by the use of an outer layer of a thin film which has twosurfaces--a polyolefin or other thermoplastic, and a surface ofethylene/vinyl acetate. The ethylene/vinyl acetate side of the filmadheres lightly to the crosslinked acrylic surface. The describedcombinations of substrate and film can survive the thermal and physicalstresses not only of the thermoforming process, but also of thepreceding lamination process, so that the film can be left on thethermoformed product and used as a protective cover which can easily beremoved at the point of ultimate use of the thermoformed product.

BACKGROUND OF THE INVENTION

Acrylic sheets are made in thicknesses of about 0.08 to about 0.500 inchand used in relatively small areas of about 5 ft.² and large area piecesof as much as 200 ft.². They are commonly thermoformed into largeconsumer and other products such as indoor and outdoor spas, tub/showerenclosures, skylights, bathtubs and highway signs. They may be used asmonolithic polymethylmethacrylate sheets or as laminates of such acrylicsheets with other thermoplastics such as ABS. Prior to the presentinvention it has been known to place a protective film on the acrylicsheet prior to thermoforming, and to employ such film as a protectivelayer which is intended to be removed only after the thermoformingprocess. Some kinds of protective materials, such as those based onpaper, are totally unsuitable for the thermoforming process since theycannot be stretched or conformed to the shape of a mold--thus, if theyare used, they can be used to protect the sheet only up to a point priorto thermoforming, where they must be removed. Frequently new protectivewrappings or coatings must be placed on the product after thermoforming.A prior commercial process and product we are familiar with employed apolyethylene film as a protective layer which ostensibly could be lefton during thermoforming. But the polyethylene film was often extremelydifficult to remove after thermoforming, and caused other problems suchas pitting in the acrylic surface due to the generation of air bubblesunder the film.

In the case of acrylic/ABS laminates such as are made by the method ofU.S. Pat. No. 5,069,851 to Hicks et al and other laminates such asacrylic/PVC laminates, it has been difficult to place the film on theacrylic--a separate step must be included after lamination to do so,since the film will frequently not survive the lamination step.

We have also been made aware of Japanese Patent Application No.61-257918, filed in Japan Oct. 29, 1986, published May 16, 1988,63-111050, which describes a film to be left on an acrylic substrateduring thermoforming. The film is a two-layer film of low-densitypolyethylene and ethylene/vinyl acetate which is 20 microns thick.Example 1 of this disclosure describes the application of the film to atwo-layer sheet of "MMA/ABS" which is 2.5 mm thick, and the subsequentvacuum molding of it in "an equipment of the male die push-up type". Thedisclosure does not provide the meaning of "MMA" and does not mentionthe temperature range, if any heating is used. While it does mentionthat the "peeling off" of the film is easier after vacuum molding ascompared to four other types of film, it does not give their thicknessesand other characteristics.

SUMMARY OF THE INVENTION

This invention is a new thermoformable sheet which includes an acrylicsubstrate of crosslinked acrylic, primarily polymethylmethacrylate, andan overlying film of coextruded low-density polyethylene andethylene/vinyl acetate. The crosslinked acrylic substrate may in turn belaminated with other thermoplastic materials such as ABS or PVC. Theinvention also includes methods of making the sheet and methods ofthermoforming it, as well as the thermoformed products made thereby.

The film we employ is a cast, coextruded film of low densitypolyethylene on one side and a blend of polyethylene and ethylene/vinylacetate copolymer on the other. It ranges from about 2 to about 4.0 milsin thickness (1 mil=0.001 inch; therefore the film is about 5 to about10 microns). The low density polyethylene layer is thicker than thepolyethylene/polyvinyl acetate-ethylene copolymer layer--generally about3:1 to about 7:1. The polyethylene may contain a small amount, usuallyabout 5%, of a finely divided clay to inhibit adhesion when the film isformed into a roll. The ethylene/polyvinyl acetate material is a blendof polyethylene to polyethylene/vinyl acetate copolymer of about 10:1 toabout 4:1, preferably about 12% EVA copolymer. The film is used in largerolls, so it is applied continuously to the acrylic sheet as the acrylicsheet is made.

The acrylic sheet substrate is cast continuously on large stainlesssteel belts of the type described in Hellsund U.S. Pat. No. 3,371,383and Opel et al U.S. Pat. No. 3,376,371 in widths from about 24 inches toabout 110 inches and thicknesses of about 0.080 to about 0.5 inch. Thecasting syrup typically includes from about 60% to about 95% methylmethacrylate and about 5% to about 40% polymerized methyl methacrylatedissolved in it to impart viscosity and to preempt the generation ofheat during the cure. Significantly for present purposes, theformulation includes about 0.1% to about 0.3% crosslinking agent. Ifthere is too much crosslinking agent, the material will be difficult tothermoform; if there is too little, it may have difficulty retaining athermoformed shape. Suitable crosslinking agents include ethylene glycoldimethacrylate, polyethylene dimethacrylate, butane, hexane and/orhigher dimethacrylate; any dimethacrylate having a linking group of upto ten carbon atoms will be satisfactory. Ideally for many purposes,about 10% to about 20% of the polymethylmethacrylate in the sheet is notcrosslinked. Thus it is clear that our products are designedspecifically for thermoforming. References herein to acrylic sheet areintended to include both opaque and clear forms; the prior art opaquesheets may have high pigment contents and perhaps other kinds offillers, as is known in the art.

The film in an appropriate width for the cast acrylic is taken directlyfrom a roll and placed on the acrylic while it is still on the castingbelt, even while the acrylic may be at temperatures as high as 150°.Adherence to the acrylic is assisted by the application of a heatedrubber roll. The product is thus a continuous cast sheet. The sheet canthen be cut into the desired lengths as in the prior art, or rolled intolarge rolls, also as is known. The more or less conventional crosslinkedacrylic may be sent directly for thermoforming where it is formed intodesired shapes in a known manner and the film is then simply peeled off,preferably at the time of ultimate installation. If the material is tobe laminated with ABS, for example, it is sent to a laminating facilitysuch as is described in the above-mentioned Hicks et al patent, and isable to withstand the heat and pressure applied in such a process. Theresulting ABS/acrylic laminate has a peelable film on it which canfurther withstand the thermoforming process and still be peelable. Theentire specification of the Hicks et al U.S. Pat. No. 5,069,851 isincorporated herein by reference.

DETAILED DESCRIPTION OF THE INVENTION

This invention will be illustrated in further detail by the followingexamples and comparative examples.

EXAMPLE 1 AND COMPARATIVE EXAMPLES 1 AND 2

To the acrylic surface of a continous cast crosslinkedpolymethylmethacrylate sheet (0.125" thick), the vinyl acetate-ethylenecopolymer side of a two-layer film of low density polyethylene/vinylacetate-ethylene copolymer (0.0025" thick) was applied as a protectivefilm to produce a vacuum-moldable sheet. This sheet was then subjectedto infra-red heating to a surface temperature of 360° F., upon whichtime it was vacuum molded using equipment of the female die pull-downtype.

For comparison purposes, the above procedure was repeated, wherein papermasking (comparative Example 1) and low-density polyethylene (0.0025"thick) (comparative Example 2) were applied to the acrylic surface, as aprotective film. All other conditions remained substantially the same.The results of Example 1 and comparative Examples 1 and 2 were as shownin Table I.

EXAMPLE 2 AND COMPARATIVE EXAMPLES 3 AND 4

A continuous cast cross-linked polymethylmethacrylate sheet (0.125"thick) was manufactured, wherein three different protective filmslow-density polyethylene/vinyl acetate-ethylene copolymer (Example 2)paper masking (comparative Example 3), and low-density polyethylene(comparative Example 4) employed above were applied to the acrylicsurface of the above-mentioned sheet. These three sheets, with theirrespective protective film applied, were then put through an extrusionprocess for the purpose of producing a composite material of acrylic andABS. During this process, an ABS layer (0.125" thick) was applied to thebottom side of the above-mentioned sheets to produce a composite with atotal thickness of 0.250" (0.125" acrylic/0.125" ABS). These sheets werethen subjected to infra2red heating to a surface temperature of 390° F.upon which time they were vacuum molded using equipment of the male diepull-down type. The results of Example 2 and comparative Examples 3 and4 were as shown in Table II.

                  TABLE I                                                         ______________________________________                                                     Elon-    Air      Peeling                                                                              Peeling                                 Protective   gation in                                                                              Entrap-  Before After                                   Film         Molding  ment     Molding                                                                              Molding                                 ______________________________________                                        Ex-    Low-      Good     No     Good   Good                                  ample 1                                                                              Density                                                                       Poly-                                                                         ethylene/                                                                     EVA                                                                    Com-   Paper     No       No     --     --                                    parative                                                                             Masking   Stretch                                                      Ex-                                                                           ample 1                                                                       Com-   Low-      Good     Yes    Fair   Poor                                  parative                                                                             Density                                                                Ex-    Poly-                                                                  ample 2                                                                              ethylene                                                               ______________________________________                                    

                  TABLE II                                                        ______________________________________                                                                           Peeling                                                                       After                                                     Peeling   Peeling   Extrusion                                                 Before    After     Process &                                         Protective                                                                            Extrusion Extrusion Vacuum                                            Film    Process   Process   Molding                                    ______________________________________                                        Example 2                                                                              Low-      Good      Good    Good                                              Density                                                                       Poly-                                                                         ethylene/                                                                     EVA                                                                  Comparative                                                                            Paper     --        --      --                                       Example 3                                                                              Masking                                                              Comparative                                                                            Low-      Fair      Poor    Poor                                     Example 4                                                                              Density                                                                       Poly-                                                                         ethylene                                                             ______________________________________                                    

We claim:
 1. An acrylic sheet having improved properties forthermoforming comprising a crosslinked acrylic sheet having adheredthereto a coextruded film comprising a polyethylene layer and a layer ofa blend of polyethylene and a copolymer of ethylene and vinyl acetate,said film being from about 0.002 inch to about 0.004 inch thick. 2.Acrylic sheet of claim 1 about 0.08 to about 0.5 inch thick.
 3. Acrylicsheet of claim 1 crosslinked with about 0.1% to about 0.3% by weight ofa dimethacrylate monomer.
 4. Acrylic sheet of claim 1 wherein the ratioof polyethylene to copolymer in said blend is about 10:1 to about 4:1 byweight.
 5. Acrylic sheet of claim 1 wherein the thickness of thepolyethylene layer of the film and the thickness of the blend layer ofthe film have a ratio of about 3:1 to about 7:1.
 6. A laminate havingimproved thermoforming properties comprising a crosslinked acrylicsubstrate having laminated to one side a layer of ABS and to the otherside a coextruded film of a layer of polyethylene and a layer of acopolymer of ethylene and vinyl acetate.
 7. A laminate of claim 6wherein the crosslinked acrylic substrate includes about 0.1% to about0.3% by weight crosslinking agent.
 8. A laminate of claim 6 wherein theratio of thickness of polyethylene to the thickness of copolymer ofethylene and vinyl acetate in the film is about 3:1 to about 7:1.
 9. Alaminate of claim 6 wherein the ratio of polyethylene to copolymer insaid blend is about 10:1 to about 4:1 by weight.
 10. A laminate of claim6 wherein the ratio of thickness of the acrylic to the thickness of theABS is about 2.5:1 to about 1:4.
 11. An article thermoformed from alaminate of claim
 6. 12. An article thermoformed from an acrylic sheetof claim 1.